HYDROCARBON

Week:                                                                                  Date:                                                    

Period:                                 Duration: 1 HR 20 MIN.                                                 Average age of learners: 16YEARS

Subject:                               CHEMISTRY                                                                        Class: SS TWO

Topic:                                    HYDROCARBON

Sub topic:  Reference materials:

(1) ESSENTIAL CHEMISTRY, TONALD PUBLISHERS, I. O ODESINA

(2) NEW SCHOOL CHEMISTRY, AFRICAN FIRST PUBLISHERS, OSEI YAW ABABIO

(3) INTERNET

Instructional materials:

Entry behavior: The students have been taught characteristic of organic compound

Behavioural objective: At the end of the lesson the students should be able to:

i.                     Define hydrocarbon and their sources

ii.                   State the classification of hydrocarbon

iii.                  Explain alkanes

iv.                 Explain the laboratory preparation of alkanes

v.                   State the physical properties and uses of alkanes

CONTENT

HYDROCARBONS

Hydrocarbons are compounds of carbon and hydrogen only.

SOURCES OF HYDROCARBON

 There are three main natural sources of hydrocarbons: natural gas, petroleum and coal.

Classification of Hydrocarbon

Depending upon the arrangement of carbon atoms in their structure, organic compounds are broadly categorized into

·         Acyclic or Open Chain compounds

·         Cyclic or Closed Chain compounds

The following diagram will give you a clear idea about the classification of organic compounds:

Acyclic or Open Chain Compounds

The carbon atoms are present in the form of an open chain. This chain may either be a straight chain or a branched chain. These were initially known as Aliphatic compounds because the compounds of this class were derived from either animal or vegetable fats

Straight Chain Compounds: The carbon skeleton is in the form of a straight chain. Examples:

n-Propane  CH₃-CH₂-CH₃

Propene      CH₂=CH-CH₃

Branched Chain Compounds: The carbon skeleton is in the form of a branched chain. Examples: Isobutylene

Cyclic or Closed Chain Compounds

They are marked by the presence of one or more closed chains or ring of atoms in their structure. Depending on whether there is a presence of any other atom apart from carbon in the constitution of the ring, they are further classified as:

Homocyclic or Carbocyclic Compounds

Heterocyclic Compounds

Homocyclic or Carbocyclic Compounds

The rings in these compounds are entirely made up of carbon atoms. No other atom is present in the ring skeleton. These can be further divided into two sub-classes:

Alicyclic Compounds and Aromatic Compounds

Alicyclic Compounds

Their name is attributed to their resemblance to Aliphatic compounds in their properties. The examples of this category include cyclopropane,  cyclobutane, cyclopentane, cyclohexane, etc.

Aromatic Compounds

These are cyclic unsaturated compounds. They derive their name from the Greek word Aroma which means “fragrant smell” since most of these compounds bear a pleasant smell. These are further classified into two types:

Benzenoid Aromatic Compounds: They are characterized by the presence of one or more fused or isolated benzene rings as well as their derivatives in their structure. Depending upon the number of benzene rings that are fused together in their structure, they can be further classified as Monocyclic, Bicyclic, Tricyclic.

Non-Benzenoid aromatic Compounds: They are characterized by the presence of a single benzene ring to which other groups are attached.

Types of bond fission
The breaking of a covalent bond is called bond fission. Reactions of organic compounds involve bond fission followed by the formation of new bonds. Two types of bond fission are possible, homolytic fission and heterolytic fission.

These are very important principles, used throughout organic chemistry.

In the example above, a covalent bond breaks so that one of the bonding electrons goes to each of A and B.

Homolytic fission forms two free-radicals.

A free radical is a species with an unpaired electron

Heterolytic fission

In the example above, a covalent bond breaks so that both the bonding electrons go to either A or B.

Heterolytic fission forms oppositely-charged ions.

NOTE: 

Homolytic fission → free radicals 

Heterolytic fission → ions

 

ALKANES

Alkanes are aliphatic hydrocarbons. They form a homologous series of saturated hydrocarbons with general molecular formula, C_nH_2n+2. Alkanes are tetrahedral in shape. All alkanes have similar name with suffix –ane . They only have single bond (–) only. The physical properties, such as melting and boiling point, density and states of matter of alkanes increase steadily with increasing molar mass. The first four members are gases, liquid appear with pentane, C₅H₁₂ and from hexadecane, C₁₆H₃₄, onward, the members are solids at room temperature.

Members of alkanes family are methane, ethane, propane, butane, pentane, hexane, heptane, octane, nonane, decane etc.

For example, hexane, a straight chained 6-carbon alkane and 2-methyl pentane, 3- methyl pentane, 2,2-dimethyl pentane are all branched 6-carbon alkane, have the same molecular formula, C₆H₁₄.

   

2,2,3-trimethylpentane    3-methylhexane         butane

 

 

                                       

4,5,5-triethyl-3,6,6-trimethylnonane   2,3,4-trimethylhexane   

                                 

 2,2,3-trimethylbutane                      4-ethylheptane

 

METHANE

Methane is the first and simplest compound  in the alkane series with the molecular formula CH4. It is a component of petroleum gas and a major constituent of natural gas. Methane is produced during decomposition of plant matter in the absence of air.

LABORATORY PREPARATION OF METHANE

Methane is prepared by heating anhydrous sodium ethanoate with an equal mass of soda-lime (soda-lime is a mixture of sodium of sodium hydroxide and calcium oxide the active reagent is the sodium hydroxide)

CH3COONa(s)   +  NaOH(s)                    CH4(g)   +  Na2CO3(s)

Note: Soda-lime is used in preference to caustic soda, (NaOH) because it is not deliquescent and does not attack glass.

 

BY THE CATALYTIC REDUCTION OF METHYL IODIDE:
                    CH₃-I + H₂   CH₄ +HI
 BY THE HYDROLYSIS OF ALUMINIUM CARBIDE:
      In laboratory methane can be prepared by boiling aliminium carbide with water.
                     Al₄C₃ + 12H₂O    3CH₄ +Al (OH)₃

Physical properties of methane

1.       It is a colorless and odourless gas.

2.       It is denser than air.

3.       It is neutral to litmus paper.

4.       It is slightly soluble in water.

Chemical properties of methane

1) Combustion

Alkanes burn in air to ALWAYS form carbon dioxide and water. 

When there is insufficient oxygen, the product is ALWAYS carbon monoxide and unburnt carbon.

Example: methane is commonly used camping gas. State the chemical equation of combustion of butane in air.
             2 CH₄ (g) + O₂ (g) -->  CO₂ (g) +2H₂O (l)

High alkanes burn less completely and gives soot (unburnt carbon) and CO

Incomplete combustion forms soot and CO. It’s produced more than alkane

 

2) Reaction with Chlorine/Other Halogens (Alkyl Halides)

Chlorine molecule replaces alkane hydrogen atom with chlorine atom

Substitution reaction – the reaction in which one or more atoms replace other atoms in a molecule

Light is needed to break covalent bond between chlorine molecule --> atoms

The reaction is catalysted by sunlight ( i.e photocatalysis).

CH_4(g)  + Cl_2(g)            CH₃Cl_(g)  + HCl_(g) (chloromethane)

CH₃Cl_(g)  + Cl_2(g)         CH₂Cl_2(g) + HCl_(g) (dichloromethane)

CH₂Cl_2(g)  + Cl_2(g)        CHCl_3(g)  + HCl_(g) (trichloromethane)

CHCl_3(g)  + Cl_2(g)          CCl_4(g)  +   HCl_(g) (tetrachloromethane)

Uses of methane

1.       It is used as fuel.

2.       It is used in the production of water gas.

3.       It is used as an anaesthetic  in surgical operation (trichloromethane)

4.       Tetrachloromethane is a very useful organic solvent in industries.

5.       It is used for making hydrogen, carbon(iv)sulphide, carbon black, trichloromethane, tetrachloromethane.

PRESENTATION

Step I: The teacher explains hydrocarbon and its classification

Step II: The teacher explains alkanes as unsaturated hydrocarbon

Step III: the teacher leads the students to derived the molecular formula of first – tenth member of alkanes family

Step IV: The teacher draws all the isomers of heptanes

Step V: The teacher explains the laboratory preparation of alkanes

Step VI: The teacher state the physical and chemical properties of alkanes

EVALUATION

The teacher evaluates the students by asking the following questions:

i.                     Define hydrocarbon and their sources

ii.                   State the classification of hydrocarbon

iii.                  Explain alkanes

iv.                 Explain the laboratory preparation of alkanes

v.                   State the physical properties and uses of alkanes

 ASSIGNMENT

Name the structure of the following alkanes